This fixes the problem when config keys collide with Kernel/Object
methods (private). An example is `exec` which is used in vagrant-exec
plugin.
Compare:
> old.send :exec
ArgumentError: wrong number of arguments (0 for 1+)
> old.public_send :exec
=> #<Vagrant::Config::V2::DummyConfig:0x007fe212cc05c8>
This is done by calling the `upgrade` method on the _old_ configuration
classes. The old configuration classes are given the complete new
configuration and can set whatever settings they need to on it.
